1. Field of the Invention:
The present invention relates to a semiconductor memory and a method of manufacturing the same, and more particularly, to a semiconductor device which has sense amplifiers, each including a pair of MOSFETs arranged in complete symmetry, and can therefore reduce variations in electric characteristics, and a method of manufacturing the same.
2. Description of the Related Art:
With increasingly higher miniaturization, integration, and performance of semiconductor memories, DRAMs have also been reduced in cell size and memory capacity has increased. In addition, the need for high performance requires the use of lower operating voltages, so that variations in the characteristics of transistors exerts an increasingly larger influence.
In the operation of a DRAM, the charge accumulated on a capacitor for each cell in the DRAM increases the potential on a bit line by a subtle value, and this subtle potential is amplified by a sense amplifier to determine data “1” or “0” which is then relied on by the device to operate. The trend of higher miniaturization and integration make it difficult to ensure the memory capacity of cells in the DRAM, and faster operations require a reduction in voltage, so that the sense amplifier is required for perform more and more important roles.
The sense amplifier is a semiconductor device formed of at least one pair of MOSFETs which are required to have a high symmetry in regard to their characteristics. While problems can arise from variations in the characteristics of the respective MOSFETs themselves, the most critical issue lies in variations in differences in respective characteristics between the pair of MOSFETs. Since a difference in these characteristics, if any, results in a signal with a reduced amount of information due to the miniaturization of DRAMs and a reduction in voltage, even a subtle difference in characteristics is more likely to affect the operation of the sense amplifier and cause malfunctions.
FIG. 1 illustrates the planar structure of a conventional general sense amplifier. FIG. 1 is a schematic plan view illustrating the arrangement of a sense amplifier for reducing variations in difference in characteristics of a pair of MOSFETs which form part of the sense amplifier, disclosed in JP-A-6-13574.
Annular gate electrodes 62a, 62b are formed on diffusion layer 61. Also, contacts 66 are formed on diffusion layer 61, contacts 67a, 67b are formed on diffusion layer 61 within gate electrodes 62, and contacts 68 are formed on gate electrodes 62a, 62b of the sense amplifier. Shallow trench isolation (STI) area 70 is defined around diffusion layer 61.
Assume that MOSFET 6a and MOSFET 6b are a pair of MOSFETs which operate as a sense amplifier. In MOSFET 6a, current flows between contact 67a and contact 66 through gate electrode 62a. In MOSFET 6b, current flows between contact 67b and contact 66 through gate electrode 62b. Since contact 67a and contact 67b are arranged in symmetry about the center of the sense amplifier on the top surface of diffusion layer 61, variations are reduced in the difference in characteristics between MOSFET 6a and MOSFET 6b. 
However, even in the semiconductor memory described in JP-A-6-13574, a constant distance cannot always be ensured from gate electrodes 62a, 62b to STI area 70 for element isolation. If gate electrodes 62a, 62b are shifted in position in the processing step for forming them on diffusion layer 61, diffusion layer spacing 64 between gate electrode 62a and STI area 70 in MOSFET 6a will differ from diffusion layer spacing 65 between gate electrode 62b and STI area 70, thus causing MOSFET 6a and MOSFET 6b of the sense amplifier to lose the symmetry.
Also, in some methods of placing the foregoing components on diffusion layer 61, STI area 70 can be formed with edges having different shapes, which connect to diffusion layer spacing 64 and to diffusion layer spacing 65, resulting in a different impurity profile.
Even at least the two aspects described above will cause a difference in characteristics of MOSFET 6a and MOSFET 6b, giving rise to a subtle difference in characteristics of a resulting DRAM. The increasing miniaturization of DRAMs and more reduction in voltages used therein give rise to a problem in which even a subtle difference, if any, affects the operation of the sense amplifier and increases the possibility of malfunctions.